1. Field of the Invention
The present invention relates first of all to a method for determining the speed of a helicopter with respect to the mass of air in which it moves as well as a system for determining this speed, which is the true air speed of the aircraft. From the aerodynamic point of view, it is the speed which is important for safety reasons especially. From the navigational point of view, it is the ground speed which should be taken into account. Without wind, it is the same. If there is a wind, it is necessary to take into account the effective wind and the cross wind and to make the necessary corrections.
In its general plane a helicopter has a longitudinal axis OX and a transverse axis OY. The blades of the helicopter forming the rotor disk, or main lift rotor, are articulated to a hub extending along an axis OZ, often perpendicular to the two axes OX and OY. The movement of the helicopter in the plane OX, OY as well as in altitude along OZ takes place by varying the pitch, or setting, of the blades of the rotor, that is to say by rotation of the blades about their foot, or about the articulation fixing them to the hub. The setting angle of the blade is the angle which the zero lift chord of its profile makes with the plane of rotation.
The setting of the blades is determined by means of a cyclic pitch control plate, or swash plate. This plate, in fact a double plate, may be driven in translation along the axis OZ by a first control and in rotation about the axes OX and OY by a piloting lever called swash lever. The swash plate is then formed of two plates, substantially joined together, the upper plate being however mounted for rotation with the rotor disk, the lower plate, in this respect, being fixed. The blades are connected to the upper plate by a pitch link fixed to its leading edge.
By rotating or slanting the swash plate about the axes OX and OY by translation thereof along the axis OZ, the setting of the blades may then be varied through the pitch links.
By actuating the first control, called "collective lever," the pilot drives the swash plate in translation along the OZ axis, this resulting in the variation of the setting of all blades simultaneously by a same angle. This is how the pilot makes the lift to vary and thus the altitude of the helicopter. By actuating the swash lever, and through a direct connection between this latter and the fixed plate of the swash plate, which connection is for example of the mechanical or hydraulic type with a gimbal or universal fitting, the pilot causes the swash plate to slant about one or other of the two axes OX and OY or both. On the side where the plates is slanted downwards, the edge of the blades is pulled downwards, which reduces their lift. The blades, mobile about their articulation, then have a tendency to drop. On the other side, it is the reverse. It is then by actuating the swash lever that the pilot orientates in space the lift of the rotor disk and thus controls the movements of his aircraft in the horizontal plane.
No instrument gives accurately the true air speed of a helicopter for every flight envelope.
2. Description of the Prior Art
The solution is known provided by the Badin anemometer or aircraft solution, which gives the modulus of a speed as a function of the difference between the static or atmospheric pressure and the total pressure acquired by a Pitot tube perpendicular to the relative wind. But the speed shown by a Badin anemometer is only equal to the true air speed if the static pressure is equal to 1013 mbar in a standard atmosphere, therefore close to the ground. As the helicopter rises in altitude, the static pressure decreases and the indicated speed becomes less than the true air speed, of the order of 1% per 600 feet from the ground. Furthermore, below a certain speed, about 40 knots, the dynamic pressure is too low and the lift rotor generates an aerodynamic flow causing an apparent air speed approximating the vertical and leading to aerodynamic angles of attack which are incompatible with the use of Pitot tubes and also disturbing the static pressure measurements.
The solution is also known which improves the preceding one for low speeds and uses a Pitot tube mounted, at the end of a long mast, on a vane, gimballed for movement about the transverse axis OY and the vertical of the tube, so that the total pressure taking tube is oriented in the direction of the local aerodynamic field. This solution is then more accurate than the first, but only at low speeds. Furthermore, it rests on a fragile vulnerable and complicated installation because of the gimbal mounting, and so unreliable.
A third solution is further known consisting in disposing, at both ends of an arm mounted for rotation on a foot, two pressure taking venturis--two Kniel tubes--whose difference is a sinusoidal function of the azimuth of the arm. The maximum value of this pressure difference and the corresponding azimuth give respectively the modulus of the air speed and the direction of the speed vector. But it is also a question here of a clumsy and fragile solution and so of only relative reliability.
The solution is also known provided by a probe, formed of two electrodes between which flows an ion flux by corona effect, situated on a mast above the hub of the main rotor. Thus the values of the longitudinal speed and of the lateral speed may be obtained by measuring the deflection of the ion flux. But it is a clumsy and difficult solution to use in a humid atmosphere.
A fifth solution, and there are still others, consists in assuming that the air speed of the helicopter along one or other of the above defined axes OX, OY is proportional to the difference between the cyclic pitch along the axis considered and the angle of inclination, or rotation, of the helicopter about the other of the two axes, namely the pitch angle, or pitch attitude, about the axis OY, and the roll range about the axis OX. It should be noted that the angle of rotation of the helicopter about the axis OZ is the yaw angle.
It is this fifth solution which is recommended in the French patent application n.degree. 2 282 644, published on the Mar. 19, 1976, with reference to the passages on page 9, lines 1-8 and 33-35.
The cyclic pitch Px along the axis OX is the angle formed, in the plane of symmetry XOZ of the fuselage of the helicopter, between the axis of the hub of the rotor and the control axis, perpendicularly to the swash plate, the cyclic pitch Py being the corresponding angle in the plane YOZ.
This fifth solution is quite satisfactory for flying conditions close to the stationary condition; the system for implementing it is furthermore simple and rapid to calibrate. However, the performances of this latter solution decrease rapidly as soon as the stationary conditions are no longer present, that is to say when the air speeds along axes OX and OY increase.
In short, all the solutions proposed up to now for determining the true air speed of a helicopter suffer from considerable limitations of their correct operating ranges, for some limitations at high speeds and for others at low speeds, and again for other limitations due to disturbances related to the local measurements in the disturbing flux of the rotor and to excessive fragility, even to practical impossibilities related to the need to measure non accessible parameters, such as the efficiency of a rotor, or difficult to measure with satisfactory accuracy, such as the position of the rotor disk, or to the need of complex and costly calibration.
Thus, the present invention provides a method for determining the air speed of a helicopter, performing in all flight envelopes of the aircraft and only using primary information measurements readily obtainable and with safety, and without using means external to the fuselage of the helicopter.
The present invention also provides an efficient method in the low speed flight range and particularly during air manoeuvers, making it compatible with the traditional anemobarometric system, so as to provide continuously and accurately the speed information in all the flight envelope of the helicopter.